The term polyolefins here refers to mostly homo and random copolymers of polypropylene (PP). These polyolefins are used for various end use applications such as storage containers, medical devices, food packages, plastic tubes and pipes etc. In polyolefins, the uniformity in arrangement of crystals upon crystallization is a necessity to provide an effective, durable, and versatile polyolefin article. In order to achieve such desirable physical properties, it has been known that addition of certain compounds which provide nucleation sites for polyolefin crystal growth during moulding or fabrication. The polymers containing such nucleating compounds crystallize at a much faster rate than virgin polyolefin. Increase in crystallization temperature results in reduced cycle times. The nucleators provide nucleation sites for crystal growth during cooling of a polyolefin from molten state. The presence of nucleation sites can also provide clarification to the targeted polyolefin if the crystallites formed are uniform and smaller than the wave length of light. Thus, nucleating agents are very important to the polyolefin industry where obtaining faster, homogeneous crystallization and clarity are some of the critical requisites. Thermal stability, solubility among nucleation compositions, possession of surface topology capable of achieving nucleation, ability to induce nucleation with high clarity at low concentrations and avoidance of haze are few more of the desired properties of ideal nucleating agents and an object for achievement of such is truly a pressing need of art.
Study of the art reveals efforts towards attainment of the said properties. Inorganic compounds like pulverized clay, silicates, alkali salts, alkaline earth salts, aluminum salts, titanium salts, metal oxides and organic compounds including γ-quinacridone, 2-mercapiobenzimidazole, sorbitol/phosphate derivatives find mention in the art. However, these have not been able to satisfactorily and comprehensively address said problems of art.
Dibenzylidene sorbitol (DBS) derivatives are effective nucleating agents for polypropylene. They are prepared by the condensation of two moles of an aromatic aldehyde with one mole of carbohydrate like sorbitol, xylitol etc. Murai et al in U.S. Pat. No. 4,429,140, Machell in U.S. Pat. No. 4,562,265, and Kobayashi et al in U.S. Pat. No. 4,902,807 have disclosed suitable methods to prepare them. Hamada et al disclosed DBS as effective nucleating and clarifying agents for polyolefin in their U.S. Pat. No. 4,016,118. A large numbers of other acetals of sorbitol and xylitol including bis(p-methylbenzylidene) sorbitol (4-MDBS) were also disclosed. Mahaffey, Jr., in U.S. Pat. No. 4,371,645 disclosed di-acetals of sorbitol having at least one chlorine or bromine substituent in the aromatic ring. Polyolefin compositions based on sorbital derivatives as nucleating and clarifying agents for PP are disclosed in U.S. Pat. Nos. 6,582,503; 6,586,007 and 6,989,154. Currently Millard 3988 [1,3-O-2,4-bis(3,4-dimethylbenzylidene) sorbitol (3,4-DMDBS) produced by Milliken Chemical Company provide excellent clarification characteristics besides nucleation for various polyolefins.
Many others commonly known compounds are kaolin, talc etc which show high polyolefin crystallization temperatures but exhibit many drawbacks for industrial applications. For example, Camphanic acid exhibits high peak crystallization temperature in PP homopolymer formulations but exhibits very poor thermal stability and plate out during processing. Sometimes sodium benzoate, NA-11, exhibit deleterious nucleating efficiency when used in combination with calcium stearate. This is due to calcium ion from the stearate transfers position with the sodium ions of the nucleating agents, rendering the nucleating agents ineffective. Non-ionic acid neutralizers, such as dihydrotalcite (DHT4-A) used in conjunction with such nucleators reduce the above effects aesthetic characteristics, haze and higher costs. Other problems include dispersion, agglomeration of nucleating agent's leads to inconsistent nucleation, stiffness and impact variation in the polyolefin. DBS derivatives exhibit plate-out at high processing temperatures and if the aromatic rings are mono-substituted, show improved thermal stability but show organoleptic properties and as a result they cannot be used in medical devices and food packaging.
To address some of these problems there is still a need in the plastics industry to discover new compounds that do not exhibit some of the above problems and provide excellent peak crystallization temperatures with polyolefins.